This invention relates to silicone elastomeric coatings. More particularly, this invention relates to transparent shatter-resistant silicone elastomeric coatings useful for retaining fragments during breakage of glass, porcelain, and ceramic materials.
Breakage of glass, porcelain, and ceramic materials into fragments presents problems in the form of sharp projectiles, spilled contents and hazardous remains. Present means to counter this package deficiency include organic coatings on glass or ceramic surfaces, metal containers, and wire cages.
Organic coatings, however, have low temperature limitations relative to the capabilities of glass or ceramic materials. As a result, such coatings tend to thermally degrade at temperatures greater than 350.degree.-400.degree. F.
Metal containers are generally undesirable because they are not transparent or translucent and would not permit viewing of the breakable object and further, as a container for a light bulb, would not permit the transmission of light.
A wire cage permits viewing of the breakable object and lets light emissions out, but has considerably less containment potential than a continuous film. In addition, the aesthetics of a wire cage are less than desirable.
The use of silicone coatings to resist shattering is also known in the art. For example, U.S. Pat. No. 3,715,232 to Audesse et al. (Audesse), discloses an incandescent lamp having 2 silicone layers. The first layer, which is disposed on the surface of the lamp, is a shatter-resistant clear silicone rubber. It is disclosed in Audesse that this layer is satisfactory in reducing or eliminating scattering of the pieces on breakage but because of the layer's soft, tacky nature, dust collects on it and adheres so firmly that the dust could not be removed. Thus, a second layer is used on the surface of the first layer and provides a hard, nontacky surface to which dust will not readily adhere and from which dust can easily be removed. At column 2, lines 57-60, it is pointed out that the cure time is 30 minutes at a temperature of 175.degree.-185.degree. C.
Thus, Audesse teaches the use of two protective layers to provide an incandescent lamp with shatter resistance and a nontacky surface. It is desirable, however, to provide a single silicone elastomeric coating which is both shatter resistant and substantially tack-free. It is further desirable to provide a shatter-resistant coating which is faster curing.
U.S. Pat. No. 3,621,323 to Thomas et al. (Thomas), discloses the use of a single shatter-resistant silicone elastomeric coating on an incandescent lamp. The coating contains an organopolysiloxane in a hydrocarbon solvent. The particular organopolysioloxane is a methylpolysiloxane containing terminal silicon-bonded hydroxy groups which is cured with a metallic salt selected from dibutyl tin dilaurate and dibutyl tin diacetate. The coating can withstand temperatures approaching 600.degree. F. for over 1,000 hours without loss of strength or elasticity. The coating adheres firmly to the glass envelope of the lamp and is substantially translucent throughout its usable life.
Thus, Thomas teaches a silicone elastomeric coating which is heat stable and substantially translucent. It would be desirable, however, to provide a shatter resistant silicone elastomeric coating which is transparent rather than translucent so as to maximize the amount of light which passes through the coating from the lamp filament.
U.S. Pat. No. 3,529,035 to Lamoreaux discloses a silicone elastomeric composition useful as a shatter-resistant coating for light bulbs, wherein the composition comprises R.sup.1.sub.2 units, SiO.sub.2 units, and --O--[--SiOR.sub.2 --].sub.n -- units, with n having a value of at least 1,000. The coating is transparent and tough, and has a cure time of 1-4 hours at 50.degree.-150.degree. C.
Although the shatter-resistant coating taught in Lamoreaux is transparent, it would be desirable to provide a transparent shatter-resistant coating which is also faster curing.
In U.S. Pat. No. 4,860,906 to Pelligrini et al. (Pelligrini), a glass container is provided with an elastomeric silicone coating having improved resistance to shock, breakage, and fragmentation and having substantial heat stability. The coating is preferably a silicone rubber made by compounding silicone elastomers with fillers and vulcanizing agents. The coating is also transparent and resists discoloring at elevated temperatures. Furthermore, the coating provides a rubber-like, non-slip surface. Pelligrini does not disclose the cure time of the coating.
Although Pelligrini teaches that silicone elastomeric coatings in general can be used as shatter-resistant coatings for glassware, it has been found that not all silicone elastomeric coatings are shatter-resistant. It has further been found that transparency and cure times vary with the silicone elastomer composition. Thus, it is continually desired to provide a shatter-resistant silicone elastomer coating which is transparent and fast curing.
The present invention is based on the discovery that a silicone elastomeric composition containing critical amounts of a resinous copolymer comprising R.sub.3 SiO.sub.1/2 units and SiO.sub.4/2 units (MQ resin) in combination with a vinyl-containing polydiorganosiloxane, a organohydrogenpolysiloxane, and a platinum-containing catalyst, will provide a single shatter-resistant coating for glass, ceramic, or porcelain articles, which is transparent, fast curing, and stable at elevated temperatures.
The combination of ingredients used in the present invention is known in the art. For example, the combination is disclosed in U.S. Pat. No. 4,528,314, to Modic, as a dirt resistant silicone coating for use on silicon-coated membrane structures. The coating is transparent in the absence of fillers.
U.S. Pat. Nos. 4,774,297 and 3,983,298, to Murakami et al. and Hahn et al., respectively, and copending, commonly-assigned application Ser. No. 339,041, filed Apr. 17, 1989, disclose pressure sensitive adhesives containing the ingredients present in the composition of the present invention.
Although the above references disclose the combination of ingredients used in the present invention, the references do not teach the combination of the ingredients containing the resinous copolymer in the amounts critical to the curing of the composition of this invention to form shatter resistant coatings which are transparent and fast curing.